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LMH6554: Open Loop Transimpedance

Masanori Tachibana
Genius 5350 points
Part Number: LMH6554


Dear support team,

Our customer is evaluating LMH6554.
The customer has designed circuits that affect the characteristics of LMH6554 open-loop transimpedance.

However, the result of simulation and the characteristics of figure 22 in the Data Sheet do not match, so he wants to know the following.
1. Could you teach me the measurement circuit diagram of Figure 22 in Data Sheet?
2. Is there a simulation model that matches the open-loop characteristics with that in Figure 22?

Best regards,
M. Tachibana

  • 0
    TI__Genius 12631 points

    Hello,

     Attached is a TINA circuit showing how to simulate the Zol of the LMH6554.The Zol doesn't exactly match the curve at very high frequencies, but it is close.

    We do not have a method to measure Zol in the lab. The curve shown in figure 22 is an IC level simulation. We can verify the curve in the figure by doing a lot of closed loop frequency response testing for different gains, feedback resistance and output loads and checking them against the simulated closed-loop results. If the closed loop results between bench and simulation match well, then we have a high degree of confidence that the simulated Zol matches reality.

    -Samir

    6523.Zol_Method2.TSC

  • 0 in reply to Samir Cherian
    Genius 5350 points
    Hello Cherian-san,

    Thanks for your reply.
    I simulated the circuit which you sent, and confirmed that it is close to the characteristic shown in Fig22.
    By the way, is there any way to actually measure the characteristics of open transimpedance (shown in Figure 22)?
    For example, from the characteristics of closed-loop circuits with different gains, is there any way to actually measure the characteristics of open loop indirectly?

    Best regards,
    M. Tachibana
  • 0 in reply to Masanori Tachibana
    TI__Genius 12631 points
    Hello Masanori-san,
    To the 1st order you can vary the feedback resistance and closed-loop gain and measure the frequency response to estimate the Zol. The complication is that there are second order effects which are problematic in the high speed world, for example the parasitics from feedback capacitance and output capacitance, and the amplifiers input capacitance. These effects no longer make the analysis simple.
    The important things to remember is that you want to stay away from where the 2nd pole occurs in the Zol curve for sufficient phase margin, so if you only want an approximation and don't care too much about the exactness then the above method makes it possible.

    -Samir
  • 0 in reply to Samir Cherian
    Genius 5350 points

    Dear Cherian-san,

    Thanks for your advice.

    I understand that the second pole is influenced by parasitic elements inside and outside the IC.
    And I also understand that it affects the phase margin.

    I'll suggest the customer that he evaluates the characteristics with actual IC.

    Best regards,
    M. Tachibana